Known in the art is an internal combustion engine provided with a plurality of cylinders arranged on a straight line, for example, an in-line 4-cylinder internal combustion engine, in which a No. 1 cylinder and No. 4 cylinder skipping one ignition timing are connected to a first exhaust manifold, a No. 2 cylinder and No. 3 cylinder skipping one ignition timing are connected to a second exhaust manifold, an outlet of the first exhaust manifold and an outlet of the second exhaust manifold are connected to a common exhaust pipe, and an air-fuel ratio sensor is arranged in this common exhaust pipe (see Japanese Patent Publication (A) No. 2001-3798). When merging the exhaust ports of the cylinders outside of the cylinder head at a single exhaust pipe using an exhaust manifolds in this way, various methods of arranging the pipes have been adopted in the past.
However, in an internal combustion engine provided with a plurality of cylinders arranged in a straight line, exhaust ports of a pair of cylinders positioned at two ends and an exhaust port of a center cylinder positioned between the pair of cylinders being merged at an exhaust merging portion formed in a cylinder head, and an exhaust outlet opening of the exhaust manifold being formed on a cylinder head side wall positioned outside of this exhaust merging portion, that is, in a so-called integral exhaust manifold-type cylinder head having the exhaust ports of the cylinders and the exhaust manifold of these exhaust ports formed in the cylinder head, the degree of freedom for arrangement of the exhaust ports of the cylinders is extremely low and the exhaust ports of the cylinders are formed so as to extend toward the exhaust outlet opening formed at the cylinder head side wall so that the exhaust gas from the cylinders is exhausted quickly from the exhaust outlet opening.
In this regard, in such an internal combustion engine, when arranging a sensor at an exhaust inlet part of an exhaust pipe connected to the exhaust outlet opening, exhaust gas exhausted from each cylinder reaches the sensor immediately after exhaust, so it is possible to use the sensor to detect any changes in the exhaust gas ingredients with a good response. However, in this case, a problem arises if using as a sensor a sensor having a sensor part liable to be damaged by deposition of moisture. For example, when using as the sensor an air-fuel ratio sensor detecting the oxygen concentration in the exhaust gas, the sensing part of the sensor is formed from zirconia, so if moisture deposits on the sensing part and the sensing part is rapidly cooled, the problem arises that the thermal reaction will cause the sensing part to end up fracturing.
That is, right after engine start when the engine temperature is low, the moisture contained in the exhaust gas exhausted from the combustion chambers sticks on the exhaust port inner wall surfaces and condenses. The condensed moisture merges to form large sized water droplets. These water droplets are splashed by the exhaust gas exhausted from the cylinders inside the exhaust manifold along the extended tubular shaped surfaces of the opening parts of exhaust ports to the exhaust manifold. In this regard, as explained above, in an internal combustion engine provided with an integral exhaust manifold-type cylinder head, the exhaust ports of the center cylinder are directed toward the exhaust outlet opening. As a result, the sensing part of the sensor is inevitably positioned in an extended tubular shaped surface of an opening part of an exhaust port to the exhaust manifold.
Therefore, in this case, the problem arises that the large sized water droplets formed on the inner wall surfaces of the exhaust ports of the center cylinder are splashed by the exhaust gas and deposit on the sensing part of the sensor and as a result the sensing part of the sensor is damaged.